1. Field of the Invention
The present invention generally relates to a window frame assembly for use in high speed aircraft and a method for installation of a window pane into a window frame when making the window frame assembly. More specifically, the present invention relates to the formation of a window assembly for a high speed aircraft to ensure that a low radar cross section (RCS) is maintained during the various conditions encountered by the aircraft.
2. Background Information
Sensors used in high speed aircraft, in order to properly function, need to be protected from the environment (e.g., wind and rain). This is normally achieved through the placement of an infrared (IR) window on the outer skin of aircraft to protect the sensors and allow them to function properly. An IR window allows radiation to pass through the window and be detected by sensors located behind the window. For high speed aircraft that require a low RCS, IR windows can present a problem. Aircraft require a low RCS to appear as a small object or not appear at all, to a radar detection system. The RCS of an aircraft is a measure of how well the aircraft absorbs radar energy or reflects radar energy in a direction away from the source of the radar energy.
Conventional IR windows placed onto aircraft for protection of sensors include an interface (or ridge) between the window and the window frame that would reflect radar energy directed at the aircraft. As a result, the aircraft's RCS would increase, thereby increasing the chances that a radar system would detect the aircraft. Early design specifications therefore used a 6" by 6" diamond shaped window to allow for proper dissipation and reflection of received radar energy due to the joint between the window pane and window frame on an aircraft. However, from a practicality standpoint, a window having such dimensions is too large to place in the surface of an aircraft because of aerodynamic and structural requirements. Minimum bend radius requirements resulted in a need to drastically alter the shape of an aircraft to account for such windows. Smaller windows could have theoretically been used, but would increase the RCS, thereby making the aircraft easily detectable by radar.
Subsequently, efforts turned to removing any interface (or ridge) between the window and the frame. For example, a smaller window was constructed and an electrically conductive epoxy was placed between the window pane and the window frame to make the window flush with the frame. FIG. 1 shows a partial view of a conventional window frame assembly 100. The frame assembly includes a window pane 110 and a window frame 130. A conductive epoxy 120 is placed along the entire radius of the window pane 110 and the window frame 130 to hold the window pane 110 to the window frame 120. Such a system has drawbacks. When an aircraft is traveling at a great rate of speed and encounters heavy rain (for example, raindrops having a diameter of approximately 2 mm with a rainfall rate of approximately 2 inches per hour), the rain erodes the conductive epoxy located between the window pane and the window frame. After approximately 10 minutes the epoxy will have been entirely eroded, and the epoxy and/or window pane will fall out of the frame thereby increasing the RCS of the airplane as well as possibly damaging sensors behind the window pane.
To account for the effect of heavy rain on the conductive epoxy, soldering of the window into the frame has been attempted. While rain encountered during flight will not erode solder, the large temperature variation that an aircraft will typically encounter (e.g., -65.degree. F. to +250.degree. F.) can cause the window pane to shatter due to the stress induced by the solder on the sides of the window pane. While soft solders have been considered, they are not a practiced solution because of their susceptibility to rain erosion.
U.S. Pat. Nos. 5,818,631 and 4,090,773 each describe the application of a protective layer material onto an IR window. However, neither of these patents address the issue of reducing the RCS of the window. Therefore, there is a need for an aircraft window frame assembly having small window panes which can provide a low RCS.